GCN Circular 20727

A. Smith (UMD) and I. Martinez (UMD) 
report on behalf of the HAWC Collaboration

HAWC was operating and our real-time all-sky GRB monitoring analysis was running at the time of the G274296 event. At the time of the event, the HAWC detector was oriented at (��, ��) = (141.4��, 19.0��), local zenith. 62% of the LIGO/Virgo CWB probability contour fall within our observable field (0-45 deg zenith angle).

We perform a real-time search for counts above the steady-state cosmic-ray background using 4 sliding time windows (0.1, 1, 10, and 100 seconds) shifted forward in time by 10% of their width over the course  of the entire observing period. Within each time window, we search the HAWC sky within 45 degrees of zenith using 2.1 deg x 2.1 deg square bins shifted by ~0.1 deg along the directions of Right Ascension and Declination. This analysis is optimized for detecting ~100 GeV photons and is sensitive to the most fluent GRBs. It did not report any significant post-trials events near the time of the gravitational-wave trigger.

After the GW trigger was reported, we re-analyzed the data within �� 60 seconds of the gravitational-wave trigger on 4 timescales (0.1,1, 10, 100 sec) with a reduced threshold to account for the reduced number of trials. No candidates were identified.

Additionally, we searched for longer duration emission by observing the candidate locations beginning from the time of the trigger as they transited the FOV, roughly 3 hrs or half of a full transit for most of the observable contour. This analysis is optimized for ~0.5-100TeV. We found no evidence of emission. The 5-sigma detection sensitivity of this search varies from roughly 1.4-3 x the flux of the Crab.

HAWC is a TeV gamma ray water Cherenkov array located in the state of Puebla, Mexico that monitors 2/3 of the sky every day with an instantaneous field-of-view of ~2 sr. (learn more http://hawc-observatory.org)